1. Field of the Invention
The present invention relates to a method and a system for transporting a flow of fluid (i.e. liquid or gaseous) hydrocarbons containing water. In the method the flow is transported through a treatment and transportation system including a pipeline.
2. Description of Related Art
The search for new oil or gas resources has now reached a stage where it is moving away from relatively easily accessible continental waters, and towards deeper waters. This trend is currently most visible in the Gulf of Mexico, but also offshore Norway, and any large oil or gas discoveries in the future are primarily expected in deep waters (xe2x89xa74-500 m). This development gives rise to several technological challenges. However, solutions based on sub-sea installations and long distance transport to already existing production and processing facilities have already been in use for some time in the North Sea, especially in connection with economically marginal fields in the vicinity of older platforms. This technology will become steadily more prevalent in new field developments for deep water, but also in the increasing number of smaller projects in already developed areas.
Traditionally, in the North Sea, use of sub-sea templates and pipeline transport of the well-stream in multiphase pipelines has been restricted to a few tens of kilometers. However, better simulation and design tools, better equipment for partial separation, as well as pumping and boosting, has now led to solutions of this kind being used with transfer distances of up to 110 km in the Gulf of Mexico.
The single most challenging problem for these future trends in oil and gas exploration, is the presence of natural gas hydrates in transport pipelines and equipment. Natural gas hydrate is an ice-like compound consisting of light hydrocarbon molecules encapsulated in an otherwise unstable water crystal structure. These hydrates form at high pressures and low temperatures wherever a suitable gas and free water are present. These crystals can deposit on pipeline walls and in equipment, and in the worst case lead to complete plugging of the system. Costly and time-consuming procedures may be needed to restore flow again. In addition to the mere economic consequences, there are also numerous hazards connected to hydrate formation and removal, and there are known instances of pipeline ruptures and loss of human lives due to gas hydrates in pipelines. Although hydrate is generally thought of as a problem mostly for gas production, there is now ample evidence that it is also a significant problem for condensate and oil production systems.
There are several available methods for dealing with hydrate problems. So far, the usual philosophy has been to take steps to avoid any hydrate formation at all. This can be achieved by keeping pressures low (often not possible from flow considerations), keeping temperatures high (usually by insulatingxe2x80x94which does not protect against shutdowns or long distances), removing the water completely (costly equipment and difficult), or by adding chemicals that suppress hydrate formation thermodynamically. Insulation is very often used, but is not sufficient alone. Chemical addition, specifically methanol (MeOH) or ethylene glycol (EG), is therefore the most widespread hydrate control mechanism in the industry today. These antifreezes expand the pressure-temperature-area of safe operation, but are needed in large quantitiesxe2x80x9450% of the total liquid fraction is not unusual in water-rich production. The use of MeOH in the North Sea may approach 3 kg per 1000 Sm3 of gas extracted. The need for such large amounts places severe demands on logistics of transportation, storage and injection in offshore facilities with a deficiency of space. The transport and injection processes for MeOH in particular, are also plagued with numerous leakages and spills.
Inhibitor chemicals of different types are not only used in the pipeline transport and processing areas, but also extensively in drilling operations and wells.
Partly due to the huge amounts and large costs involved in using traditional inhibitors like MeOH, there has over the last decade been extensive efforts devoted to finding chemicals which may be effective at controlling hydrates at much lower concentrations.
Many oil companies and research institutes have contributed to this effort, and at present, the results are divided into three main categories: kinetic inhibitors, dispersants, and modificators. Kinetic inhibitors have an affinity for the crystal surface, and thereby can be used to prevent hydrate crystal growth. Dispersants act as emulsifiers, dispersing water as small droplets in the hydrocarbon liquid phase. This limits the possibilities for hydrate particles to grow large or to accumulate. The modificators are to a certain extent a combination of the two other methods, attaching to the crystal surface, but also functioning as a dispersant in the liquid hydrocarbon phase. These methods have been somewhat successful, although there are practical drawbacks to most of them. The most significant problem, however, seems to be that all the best chemical additives thus far produced have significant negative environmental effects, and that no solution to this problem seems imminentxe2x80x94at least in the open literature.
There is growing understanding in the oil and gas industry that hydrate particles in a flow situation are not necessarily a problem per se. If the particles do not deposit on walls or equipment, and do not have a large impact on flow characteristics (i.e. their concentration is not too large), they simply flow with the rest of the fluids, without creating a problem situation. The challenge will therefore be to achieve this situation in a controlled manner, and making sure that hydrate formation does not take place randomly throughout the flow system.
Another aspect which will definitely be affected by the present invention, is corrosion in sub-sea pipelines. Huge sums of money and large resources in material and time are involved in protecting pipelines from corrosion, e.g. through conservative design (pipeline wall thickness, steel quality) and through the use of corrosion inhibitors. Not necessarily used in the same amounts per pipeline as the hydrate inhibitors, the total amounts of chemicals (sometimes with environmentally highly adverse effects) are huge, as they are used in such a great number of pipelines. Much of this corrosion is connected with free water, and successful results of the present invention may reduce this problem significantly.
The present invention provides a method for transporting a flow of fluid hydrocarbons containing water through a treatment and transportation system including a pipeline. According to the invention the flow of fluid hydrocarbons is introduced into a reactor where it is mixed with particles of gas hydrates which are also introduced into the reactor, the effluent flow of hydrocarbons from the reactor is cooled in a heat exchanger to ensure that all water present therein is in the form of gas hydrates. The flow is then treated in a separator to be separated into a first flow and a second flow, said first flow having a content of gas hydrates is recycled to the reactor to provide the particles of gas hydrates mentioned above, and the second flow is conveyed to a pipeline to be transported to its destination.
The flow of fluid hydrocarbons will normally come from a drilling hole well and will be relatively warm and will be under pressure. It is generally preferred to cool the flow of fluid hydrocarbons in a first heat exchanger before introducing The flow into the above-mentioned reactor.
It is sometimes desirable to add certain chemicals to the flow upstream to the reactor.
Before the flow enters the reactor it may advantageously be subjected to a mixing operation in order to disperse the water present as droplets in the fluid hydrocarbon phase.
The second flow from the separator may be mixed with wet gas in a mixing vessel before the flow is conveyed to the pipeline for further transport.
The method is particularly applicable in those cases where transportation takes place at a relatively low temperature, both on land in a cool climate and at the sea bottom.
When the surroundings are rather cool, one or more of the heat exchangers used may be an uninsulated pipe. When the surrounding temperature is sufficiently low, this will provide satisfactory cooling without any further cooling medium.
The invention also provides a system for treatment and transportation of a flow of fluid hydrocarbons containing water. The system includes the following elements listed in the flow direction and connected with each other so that the hydrocarbons may pass through the entire system (the numerals in parenthesis refer to the enclosed drawings which serve as illustration only):
connection to a hydrocarbon source (1),
a first heat exchanger (4),
a reactor (6),
a second heat exchanger (7),
a separator (8), and
a pipeline (13);
and in addition a line (9) which leads from the separator (8) to the reactor (6) and is provided with a pump (10) adapted to recycle material from the separator (8) back to the reactor (6). The pump may be any kind of pump, but it may advantageously be of a type which crushes the hydrate particles into more and smaller particles with a larger total crystal surface.
The inside of the system, in particular the inside of the reactor may be coated with a water repellent material. Tubing may also advantageously be provided with such a coating material.
The system preferably includes a mixer or a choke (5) upstream to the reactor (6).
In many cases it is advantageous to add different chemicals to the flow of hydrocarbons, in particular during start-up and when changes are made in the operation. The system accordingly contains for such purpose means for adding chemicals to the flow.